1. Field of the Invention
The present invention is directed generally to optical fiber for telecommunications and more specifically to an optical fiber capable of multimode operation at wavelengths below about 1260 nm and single mode operation at wavelengths above about 1260 nm.
2. Technical Background
Multimode fiber suitable for operation at 850 nm has been the preferred fiber for wiring homes and small businesses because the various system components (e.g. lasers, receivers) used in conjunction with this fiber are inexpensive. However, conventional 850 nm multimode fiber has undesirably low bandwidth and can support a relatively low bit rate. Furthermore, conventional 850 nm multimode fiber is incompatible with higher bit rate components, such as 1300 nm single mode lasers and receivers. Thus, both components and fiber typically must be replaced to upgrade to a higher bit rate system.
Known experimental fibers capable of both multimode operation at 850 nm and single mode operation at 1300 nm have been reported in the literature, however, those fibers had step index profiles and tended to have very low bandwidth at 850 nm.
U.S. Patent Publication No. 2003/0161597 (“ ‘597 Publication”) discloses an optical fiber capable of multimode operation at 850 nm and single mode operation at 1300 nm. The embodiments disclosed in Table I the ‘597 Publication have bandwidths of 0.6 to 1.5 GHz.km and would only be capable of transmitting 10 Gb/s signals along distances of 60 to 150 m, lower than needed in most local area networks.
Intermodal noise is a problem typically associated with multimode fibers. Intermodal noise is related to a variation of the optical intensity at a given optical fiber output location due to optical interference between modes of different phase. Many factors may act singly or in combination to produce phase changes that can cause intermodal noise. Some factors include: changes in temperature; mechanical distortions (including movement or vibration); and, changes in optical source wavelength.
Intermodal noise is a common problem in multimode fibers when used with highly coherent light sources, e.g., lasers, because the relative coherence of the modes allows the modes to affect the intensity of the light by interfering with each other. Less coherent sources, such as LED's, have a short coherence length and therefore are only subject to intermodal noise in very short lengths of fiber. However, LED sources are polychromatic and cause significant pulse broadening in the fiber, and pulse broadening reduces bandwidth. Therefore, it would be advantageous to have a fiber designed for operation with coherent light sources which does not suffer from intermodal noise.